In recent years, fiber lasers have attracted attention in various fields such as a processing field, an automobile field, a medical field, and the like. This fiber laser is superior to conventional laser devices (for example, a carbon dioxide gas laser device) in terms of beam quality and light condensing property. Therefore, compared to conventional laser devices, the fiber laser has superior features in that a time required for processing can be shortened, energy saving can be achieved, high-quality microfabrication can be easily performed, and the like. Further, since fiber lasers do not require components for spatial optics, fiber lasers have advantages in that alignment or the like is unnecessary, maintenance is unnecessary, or the like.
On the other hand, when reflected light (for example, reflected light from a processing surface of a workpiece) returns to the fiber laser, an oscillation state of the fiber laser becomes unstable. As a result, there is a likelihood that the power of output light fluctuates and processing properties deteriorate. Further, when such unstable oscillation develops to random pulse oscillation, failure in an excitation light source, breakage of fiber, or the like may be caused and the possibility of failure in the fiber laser is also conceivable. Therefore, it is necessary to monitor the power of reflected light to prevent the oscillation state from becoming unstable.
The following Patent Document 1 discloses a conventional device and method for monitoring the power of reflected light. Specifically, the following Patent Document 1 discloses a device that monitors the power of reflected light or the like, which includes a first detector for detecting output light leaked at a fusion-splicing point of an optical fiber and a second detector for detecting reflected light leaked at a fusion-splicing point of the optical fiber, by performing a calculation of excluding an influence of the output light from the detection result of the second detector using a relationship between detection results of the first detector and the second detector obtained in advance under a condition in which reflected light is not generated.